Conductive polymers have a high conductivity, which are used as solid electrolytes of a solid electrolytic capacitor such as aluminum capacitors and tantalum capacitors.
As a conductive polymer used therefor, ones are often used which are synthesized by subjecting thiophenes or the derivatives thereof to chemical oxidation polymerization or electrolysis oxidation polymerization.
In carrying out the chemical oxidation polymerization, an organic sulfonic acid is generally used as a dopant, and ammonium persulfate, hydrogen peroxide, and transition metal salts are generally used as an oxidant. Generally, in case where a process of a chemical oxidation polymerization is used to synthesize a conductive polymer, especially by using transition metal salts as an oxidant, it needs to remove unnecessary components by washing.
In such a case, a washing liquid such as an alcohol, water or the combination thereof can be generally used. However, such a washing process causes the dopant to be de-doped, decreasing the electric conductivity of the conductive polymer. In order to avoid the de-doping, it has been proposed to wash it with either or both of an aqueous solution and an ethanol solution, as previously prepared, including an organic sulfonic acid as a dopant.
However, in such a case, it is necessary to further wash it with water or an alcohol in order to avoid an excessive amount of an organic sulfonic acid from remaining. A conductive polymer that an excessive amount of an organic sulfonic acid remains is characterized in being smaller in the initial resistance and less changeable in the electric conductivity when it is kept in a hot condition for an extended period, than ones that an excessive organic sulfonic acid is removed by washing with water or an alcohol at the final stage. However, the conductive polymer in which an excessive amount of an organic sulfonic acid remains is apt to generate SO3 that is toxic when it is kept in hot condition than the conductive polymer which is washed to remove an excessive amount of an organic sulfonic acid, because the excessive (free) organic sulfonic acid is apt to be decomposed in a hot condition. Also, when a capacitor using a conductive polymer including an excessive amount of an organic sulfonic acid is used as a cathode layer, the properties thereof could be instable. (See Japanese Laid-open Patent Publication No. 10-12498)